In a simple refractive lens, the focal length f is determined by the curvatures and thickness of the lens and by the refractive index n of the optical material from which the lens is formed. However, the refractive index of all known optical materials is not a constant, but varies with wavelength, with shorter wavelengths (higher frequencies) resulting in a higher effective refractive index. This phenomenon is known as "dispersion" of refractive index, and is typically specified in terms of Abbe's V-number (otherwise known as "inverse relative dispersion"). An exemplary definition of V-number for optical glasses used in the visible spectrum is: EQU V.sub.d =(n.sub.d -1)/(n.sub.F -n.sub.C)
where n.sub.d is the index of refraction at a predetermined intermediate (yellow-green) visible wavelength "d" (587.56 nm), n.sub.F is the index of refraction at a predetermined short (blue) visible wavelength "F" (486.13 nm), and n.sub.C is the index of refraction at a predetermined long (red) visible wavelength "C" (656.27 nm). By analogy, the V-number can also be defined for other spectra and other wavelengths.
As a result of refractive index dispersion, a simple lens will be in focus at only one wavelength, and will experience chromatic aberration at all other wavelengths. It is known that a lens can be made from more than one element, with each element being made of a different material having different refractive properties. By appropriate selection of the material of each element, the radius of curvature of each surface, and of the spacing between adjacent surfaces, it is possible to manipulate various optical properties such as field curvature, distortion, and chromatic aberration. Since different optical materials have not only different refractive indices but also different relative dispersion characteristics, the dispersion effects (first order chromatic aberration) of the combined elements can effectively cancel each other over a range of wavelengths. Known lens designs thus include "achromatic lenses" which have the same design focal length at both ends of the visible spectrum (for example, F and C) and only slight (second order) deviations from the design focal length throughout the entire visible spectrum.
Another measure of the refractive index dispersion characteristics of an optical material is "partial relative dispersion" (the P-number), which measures the linearity of the dispersion relative to the wavelength: EQU P.sub.C,s =(n.sub.C -n.sub.s)/(n.sub.F -n.sub.C)
where "C" and "F" are as defined above and "s" refers to Fraunhofer's "s" line (852.11 nm), although the P-number can also be defined for other spectra and other wavelengths. For the majority of optical materials usable in the visible waveband, the relationship between the V number and the P-number can be approximated by a same linear relationship (the "normal line"), so that the V-number of a given material may be readily approximated from its P-number and vice versa. Schott crown glass type K5 and flint glass type F2 may be used as the two points defining that normal line. Optical materials for which the normal linear relationship is not valid are said to have "anomalous dispersion". Glass type FK is an example of an anomalous glass lying above that normal line, while glass type KzF is an anomalous glass lying below that line.
By forming a refractive lens from at least three different materials, including at least one having "anomalous dispersion" characteristics, it is possible to independently manipulate the focus position at an intermediate point in the spectrum of interest, making possible a so-called apochromatic or "process" lens whose optical performance is optimized at three wavelengths (for example, F, d, and C) and which minimizes second order chromatic aberrations. Refractive apochromatic lens systems utilizing at least one element formed from a glass having anomalous dispersion characteristics are also known for use in the 8-12 .mu.m infra-red range (Carl Zeiss) and for simultaneously focusing infra-red radiation throughout the 3 to 13 .mu.m on a common focal plane (Texas Instruments).
Since many glasses are available for use in the visible spectrum with normal dispersion characteristics, software is commercially available which treats n and V as independent variables which may be continuously manipulated until an optimal design is achieved, using the known fixed linear relationship between V and P to compute n for each wavelength of interest, so that selection of a particular glass (or other optical material) can be deferred until the optimal values for n and V have been computed. However, in other wavebands the available choices are much more limited (for example, at 10 .mu.m, only about 6 practical materials exist), and such a design approach is not feasible. Moreover, even in the visible waveband, there are only a relatively few available choices having anomalous dispersion characteristics. As a result, it is not practical to treat refractive index and dispersion as continuous variables, and if anomalous dispersion is used to minimize second order chromatic aberration, the particular dispersion characteristic (or the particular glass type) must be specified by the lens designer before the optimization program is able to compute the effective index for other wavelengths.
In a typical laser range finder, the line of sight ("LOS") of a high quality visual optics path used for locating and identifying the target is aligned with the line of sight of a second optical path associated with an eyesafe frequency laser used for determining the distance to the target. In use, each of the two LOS paths must be steered from its respective nominal position so that they may both be optically aligned with an externally defined system boresight, for example the barrel of a weapon or the origin of a two dimensional display. Since only the visual optics path is used for verifying the system boresight, the two LOS are manipulated in unison in a manner which ensures that both LOS paths have the same deviation from their nominal value. In a known opto-mechanical system for providing equal LOS deviation for both the visual optics path and the laser optics path, a pair of rotatable thin achromatic prisms (wedges) steered the LOS equally for the visual and laser paths. Achromatic two component (crown-flint) wedges were required since simple wedges caused the visual and laser optical paths to lose common boresight as the wedges were manipulated. Moreover, rotation of one wedge relative to the other affected both the magnitude and the direction of the boresight and was confusing to the user accustomed to independent manipulation of the Cartesian (x-y) coordinates of the boresight.